EPRs are produced commercially in solution and slurry processes with soluble vanadium catalysts. These processes are expensive to run because they require solvent removal and steam stripping steps.
The production of EPRs in a fluidized bed in a gas phase reaction process is advantageous because of the absence of large volumes of solvent or liquid monomer, the high heat removal capacity of the fluidized bed, the granular nature of the polymer which facilitates purging and post treatment, and the ability to control reaction concentrations over a wide range of conditions without limitations imposed by the solubility of reactants and catalysts.
However, a gas phase reaction process requires that the granular resin product be free-flowing and non-sticky. EPRs are often referred to as sticky polymers because they agglomerate forming sticky, granular resin bed particles under polymerization conditions or upon the cessation of gas phase polymerization. A sticky polymer has been defined as a polymer which, although particulate at temperatures below the sticking or softening temperature, agglomerates at temperatures at or above the sticking temperature. The sticking or softening temperature has been defined as the temperature at which fluidization ceases due to the agglomeration of particles of polymer in a fluidized bed. The agglomeration may be spontaneous or occur on short periods of settling.
Also, a polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers have also been referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not to flow out of the relatively small openings in the bottom of the product discharge tanks or purge bins. Polymers of this type can show acceptable fluidity in a gas phase fluidized bed reactor; but, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form, and the bed will not re-fluidize.
Stickiness is even more critical with EPRs having a crystalline content of less than 15 percent by weight. Commercially desirable EPMs and EPDMs contain about 20 to about 55 percent by weight propylene and have a Mooney viscosity of about 20 to about 120. EPDMs additionally contain about 2 to about 15 percent by weight of a non-conjugated diene to further contribute to stickiness. Further, EPRs are practically amorphous and they have glass transition temperatures of minus 50.degree. C. to minus 60.degree. C. At temperatures above the glass transition temperature, EPMs and EPDMs are rubbers whose viscosity decreases, like all rubbers, exponentially with increases in temperature. The viscosity decrease with rising temperatures is a major obstacle in fluidized bed production of EPR because agglomeration increases as particle surface viscosity decreases. At temperatures above about 30.degree. C., amorphous EPM particles become so sticky that fluidized bed polymerization cannot be carried out reliably. Particles comprising resin of lower molecular weight or lower Mooney viscosity are stickier than particles comprising resin of higher molecular weight or higher Mooney viscosity, and EPDM particles particles are even stickier than EPM due to the presence of soluble dienes.
Industry has generally solved tiffs problem of sticky polymers by avoiding polymerization at temperatures at or above the sticking temperature of the polymer. Such polymerization processes are disclosed, for example, in U.S. Pat. Nos. 5,087,522 and 5,208,303 and WO 88/02379.
U.S. Pat. Nos. 4,994,534 and 5,304,588 describe a method for overcoming the tendency of EPRs to become sticky during polymerization. In these patents, EPRs are produced in a fluidized bed using an alpha olefin polymerization catalyst such as a transition metal catalyst of vanadium and/or titanium and an inert particulate material. The EPRs are produced by polymerizing at or above the softening or sticking temperature of the polymer product. While this procedure is extremely effective and permits operation of the fluid bed above the sticking temperature of the polymer, it generally results in the final polymer product containing a relatively large amount (about 10 to 50 percent based on the total weight of the final polymer product) of the inert particulate material.
Use of inert particulate materials as fluidization aids in the amounts needed for fluid bed operations can also impart undesirable properties (mechanical or melt compounding difficulties) to the final product making them unsuitable for some end uses. For example, silica can be highly abrasive to the mixing equipment normally used with EPDM resins and can retard sulfur vulcanization. Carbon black always produces a black product, so that it cannot be used in markets in which a colorable resin is desired, such as rubber mechanical goods for some automotive use, oil viscosity improvers, or molded articles for consumer use. Furthermore, the inert particulate materials require storage vessels, feeding devices, special treatment to ensure dryness, and the use of additional co-catalyst beyond that required by the catalyst, all of which impose additional costs on the process.
U.S. Ser. No. 029,821 filed Mar. 11, 1993, now U.S. Pat. No. 5,376,793, describes another method for overcoming the sticky tendency of EPRs at or above the sticking temperature of the product resin by using a non-sticky prepolymer prepared by a prepolymerization conducted in a hexane slurry of ethylene and optionally a comonomer such that the comonomer content of the prepolymer ranges from about 0 to about 15 weight percent based upon the total weight of the monomers. According to the described process an inert particulate material can be incorporated into the prepolymer or introduced directly into the fluidized bed reactor independently of the prepolymer. Attempts to prepare EPR compositions high in propylene and low in molecular weight by this procedure results in a significant amount of prepolymer residue (up to 20 percent by weight based on the weight of the final polymer product) remaining in the product to prevent agglomeration during polymerization of the final polymer resin. Or, alternatively, substantial quantities of inert particulate material are added to the reactor during polymerization of the resin product. This procedure can result in undesirable levels of inert particulate material or catalyst residue in the product that can diminish final polymer product properties. Furthermore, the prepolymer residues in the product contain sufficient material that is still crystalline at temperatures as high as 120.degree. C. such that properties of the final product can be adversely affected and end use applications limited.
Thus, a process which produces a non-agglomerating or non-sticky colorable resin, without feeding an inert particulate material to the polymerization reactor, having little or no inert particulate material in the final product, and still permitting operation of a gas phase reactor above the sticking temperature of the polymer being made, would be highly desirable. There is also an on-going need for amorphous, colorable sticky polymers, such as EPRs, produced at temperatures at or above their sticking temperatures in a gas phase polymerization, while at the same time reducing or eliminating the use of inert particulate matter. By colorable is meant that the resin is capable of producing an end product which accepts and/or exhibits a color other than black.